In recent years, silicon epitaxial wafers manufactured by vapor-phase growing silicon films on silicon wafers have been used as substrates for imaging devices such as CCDs and CISs. In such epitaxial wafers for imaging devices, it is essential to reduce the level of a heavy-metal impurity in each wafer. This is because such a heavy-metal impurity existing in a wafer causes a defect called a white flaw (white spot).
In general, when an epitaxial wafer is manufactured, an epitaxial film is vapor-phase grown at a high temperature. Therefore, if a metal impurity is present in a chamber of a vapor phase growth apparatus when the epitaxial film is grown, the manufactured epitaxial wafer is contaminated by the metal impurity. As possible sources of such metal contamination, for example, a metal impurity attached to the vapor phase growth apparatus during maintenance (cleaning) thereof, a metal impurity contained in a material of the chamber, and a stainless steel component generally used for the apparatus and a piping system are conceivable in addition to a silicon crystal and a silicon-containing compound used as raw materials.
Heretofore, a method of evaluating cleanliness (degree of contamination) of a vapor phase growth apparatus has been known in which a metal impurity in an epitaxial wafer is measured, and cleanliness of a vapor phase growth apparatus in which the epitaxial wafer has been manufactured is evaluated based on the result of the measurement (refer to Patent Document 1, for example). In the method of Patent Document 1, a metal impurity in a silicon wafer is measured by a wafer lifetime (hereinafter sometimes abbreviated as WLT) method. As a typical example of the WLT method, a microwave photo conductivity decay (hereinafter abbreviated as a μ-PCD) method based on minority carrier lifetime has been known. In this method, for example, light is applied to a sample (substrate), and the lifetime of minority carriers caused by the light is detected based on change in reflectivity of a microwave, thereby to evaluate a metal impurity in the sample.
When a metal is taken into a wafer, the WLT value of the wafer is reduced. Therefore, metal contamination in an annealing furnace or a vapor phase growth apparatus can be managed by measuring and evaluating the WLT value of a wafer annealed in the annealing furnace or a wafer grown in the vapor phase growth apparatus. In other words, it is possible to determine whether an annealing furnace or a vapor phase growth apparatus, which is used in actual processing, is contaminated by a metal impurity, by preparing a wafer for contamination management, annealing the wafer in the annealing furnace or the vapor phase growth apparatus, and measuring the WLT value of the annealed wafer.